if (this->m_carArray[i] == -1) will never be true.

// Win32Project1.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"

class __declspec(dllexport) CLoadedCarGroup // size varies.
{
public:
    unsigned __int16 m_carArray[];

    // Counts members
    int __thiscall CountMembers()
    {
        unsigned int i = 0;

        do
        {
            if (this->m_carArray[i] == -1)
                break;

            i++;
        } while (i < 23);

        return i;
    }
};


int main(int argc, char* argv[])
{
    return 0;
}